A variety of fuels are available for aircraft engines. For certain purposes, cryogenic fuels, such as liquid hydrogen, liquid methane, etc., are particularly desirable because of the large amount of cooling capability stored in the cryogenic fuel itself. For example, at sea level, atmospheric pressure, gaseous hydrogen must be cooled to a temperature of approximately 36.5.degree. RANKINE (36.5.degree. F. above absolute zero) before the hydrogen gas will condense into a liquid state. Substantial heat energy must therefore be removed from the hydrogen gas. Similarly, other cryogenic fuels have this advantageous property. The potential cooling capacity represented by this removed heat energy can be used to perform useful work in a cryogenic engine. In many conventional, air-breathing, cryogenic engines, the potential cooling capacity of liquid hydrogen represented by the heat energy removed, is used to produce liquid air (primarily liquid nitrogen and liquid oxygen) from gaseous intake air of the engine. The liquid air can then be used as an oxidizer when combined with liquid hydrogen during combustion. Thus, the aircraft need only carry liquid hydrogen as a fuel.
FIG. 1 illustrates a liquid air cycle, cryogenic engine which is especially advantageous for transatmospheric travel. That is, the engine shown in FIG. 1 is capable of operating from ground level through the atmosphere and above the atmosphere. In contrast, the cryogenic engine shown in FIG. 1 of U.S. Pat. No. 3,557,557, to Prachar, describes a turbine-type cryogenic engine which is useful for atmospheric travel.
The liquid air cycle engine 10 of FIG. 1 uses a rocket motor-type combustion chamber 12 to which is delivered liquid air by a liquid air pump 14 and liquid hydrogen or other cryogenic fuel by a liquid pump 16. Cryogenic fuel is drawn from a storage tank 18 to a condenser 20 and pre-cooler which initially cools the gaseous air 24 from an inlet 26 of the engine to form liquid air 28 at an outlet 30 of the liquefied air sump. Engines of this type are particularly advantageous for transatmospheric travel in that the pre-cooler and condenser can be designed with sufficient surface area, and the hydrogen or other cryogenic fuel to liquid air oxidizer ratio selected such that the engine produces more liquid air than it burns. The excess liquid air can be stored in a separate liquid air storage tank (not shown) for use after the vehicle has traversed the atmosphere into outer space. In addition, the engine design is highly advantageous in that the combustion chamber 12 can be physically removed from the condenser and pre-cooler section of the engine, allowing substantial flexibility in the design of the vehicle.
Typically, cryogenic engines of the type described above use a heat exchanger to transfer heat energy from the gaseous inlet air to the stored liquid hydrogen fuel. A common problem during operation of engines of this type is icing of the heat exchanger, which results when water vapor in the gaseous air (humidity) contacts the heat exchanger. Icing reduces the efficiency of the heat exchanger and can result in complete failure of the engine.
U.S. Pat. No. 3,557,557, to Prachar, illustrates one prior art technique for "freezing out" water vapor from the gaseous inlet air before this air is introduced into a flow-type heat exchanger in which liquid hydrogen fuel flows around the outside of airflow tubes. Gaseous air passes through the tubes to be liquefied. Heat energy is transferred from the gaseous air, through the tubes, to the liquid hydrogen. However, before entering the airflow tubes, the gaseous air is sprayed with a fine mist of liquefied air (drawn from a supply of liquefied air already produced by the engine). The spray of liquefied air reduces the temperature of the gaseous inlet air below the freezing point of water, producing very fine ice crystals. In theory, the ice crystals will pass through the airflow tubes without the ice crystals adhering thereto. Tests have shown that the ice crystals do adhere to the condenser tubes, eventually blocking the heat exchanger.
Other conventional attempts to prevent icing of the heat exchanger include spraying the heat exchanger with an alcohol/glycol mist. Typically, this treatment only results in ice depositions forming further down in the heat exchanger. Furthermore, the introduction of alcohol/glycol into the combustion chamber of the engine with the liquefied air is undesirable.
Another prior art technique which has been discussed in the aerospace industry is to position a rotating belt made of a fine mesh screen between the engine inlet and the heat exchanger. A liquid air mist, similar to the mist used in the Prachar U.S. Pat. No. 3,557,557, is used to pre-cool the air ahead of the rotating screen belt below the freezing point of water. The rotating screen belt then traps the air crystals. A portion of the rotating screen is moved to an area outside of the airflow chamber defined by the engine to be removed by air jets, mechanical brushes and/or scrapers. Tests have shown that this technique is ineffective in removing the ice crystals as the ice crystals have a consistency similar to that of butter. The brushes merely spread the crystals along the screen, which quickly becomes clogged. In addition, an aperture must be provided in the airflow chamber for extension of the rotating belt therethrough. This is highly undesirable; and even if it were possible to remove the ice crystals, such as by air blasts, reentry of the crystals into the air stream of the engine occurs through the aperture in the airflow chamber.
Therefore, a need exists for method and apparatus which will successfully prevent formation of ice in the heat exchanger of a cryogenic engine without introducing undesirable chemicals into the liquid air oxidizer. The apparatus should be lightweight and should not degrade performance of the engine.